..... Click the link for more information. in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought.
Anatomy and Function
Occupying the skull skull, the skeletal structure of the head, composed of the facial and cranial bones. The skull houses and protects the brain and most of the chief sense organs; i.e., the eyes, ears, nose, and tongue.
..... Click the link for more information. cavity (cranium), the adult human brain normally weighs from 2 1-4 to 3 1-4 lb (1–1.5 kg). Differences in weight and size do not correlate with differences in mental ability; an elephant's brain weighs more than four times that of a human. In invertebrates a group of ganglia or even a single ganglion may serve as a rudimentary brain.
By means of electrochemical impulses the brain directly controls conscious or voluntary behavior, such as walking and thinking. It also monitors, through feedback circuitry, most involuntary behavior—connections with the autonomic nervous system enable the brain to adjust heartbeat, blood pressure, fluid balance, posture, and other functions—and influences automatic activities of the internal organs. There are no pain receptors in brain tissue. A headache is felt because of sensory impulses coming chiefly from the meninges or scalp.
Anatomically the brain has three major parts, the hindbrain (including the cerebellum cerebellum (sĕr'əbĕl`əm), portion of the brain that coordinates movements of voluntary (skeletal) muscles.
..... Click the link for more information. and the brain stem brain stem, lower part of the brain , adjoining and structurally continuous with the spinal cord. The upper segment of the human brain stem, the pons, contains nerve fibers that connect the two halves of the cerebellum .
..... Click the link for more information. ), the midbrain, and the forebrain (including the diencephalon and the cerebrum). Every brain area has an associated function, although many functions may involve a number of different areas. The cerebellum coordinates muscular movements and, along with the midbrain, monitors posture. The brain stem, which incorporates the medulla and the pons, monitors involuntary activities such as breathing and vomiting.
The thalamus thalamus (thăl`əməs)
..... Click the link for more information. , which forms the major part of the diencephalon, receives incoming sensory impulses and routes them to the appropriate higher centers. The hypothalamus hypothalamus (hī'pəthăl`əməs)
..... Click the link for more information. , occupying the rest of the diencephalon, regulates heartbeat, body temperature, and fluid balance. Above the thalamus extends the corpus callosum, a neuron-rich membrane connecting the two hemispheres of the cerebrum.
The cerebrum, occupying the topmost portion of the skull, is by far the largest sector of the brain. Split vertically into left and right hemispheres, it appears deeply fissured and grooved. Its upper surface, the cerebral cortex, contains most of the master controls of the body. In the cortex ultimate analysis of sensory data occurs, and motor impulses originate that initiate, reinforce, or inhibit the entire spectrum of muscle and gland activity. The parts of the cerebrum intercommunicate through association tracts consisting of connector neurons. Association neurons account for approximately half of the total number of nerve cells in the brain. The tracts are believed to be involved with reasoning, learning, and memory. The left half of the cerebrum controls the right side of the body; the right half controls the left side.
Other important parts of the brain include the pituitary gland pituitary gland, small oval endocrine gland that lies at the base of the brain . It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system ).
..... Click the link for more information. , the basal ganglia, and the reticular activating system (RAS). The pituitary participates in growth regulation. The basal ganglia, located just above the diencephalon in each cerebral hemisphere, handle coordination and habitual but acquired skills like chewing and playing the piano. The RAS forms a special system of nerve cells linking the medulla, pons, midbrain, and cerebral cortex. The RAS functions as a sentry. In a noisy crowd, for example, the RAS alerts a person when a friend speaks and enables that person to ignore other sounds.
Nerve fibers in the brain are sheathed in a near-white substance called myelin and form the white matter of the brain. Nerve cell bodies, which are not covered by myelin sheaths, form the gray matter. The billions of nerve cells in the brain are structurally supported by the hairlike filaments of glial cells. Smaller than nerve cells and ten times as numerous, the glia account for an estimated half of the brain's weight. Cranial blood vessels in the brain have certain selective permiability characteristics that largely constitute the "blood-brain barrier." The entire brain is enveloped in three protective sheets known as the meninges meninges (mĭnĭn`jēz), three membranous layers of connective tissue that envelop the brain and spinal cord (see nervous system ).
..... Click the link for more information. , continuations of the membranes that wrap the spinal cord spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column , technically known as the vertebral column.
..... Click the link for more information. . The two inner sheets enclose a shock-absorbing cushion of cerebrospinal fluid.
Neural Pathways
Sensory nerve cells feed information to the brain from every part of the body, external and internal. The brain evaluates the data, then sends directives through the motor nerve cells to muscles and glands, causing them to take suitable action. Alternatively, the brain may inhibit action, as when a person tries not to laugh or cry, or it may simply store the information for later use. Both incoming information and outgoing commands traverse the brain and the rest of the nervous system in the form of electrochemical impulses.
The human brain consists of some 10 billion interconnected nerve cells with innumerable extensions. This interlacing of nerve fibers and their junctions allows a nerve impulse to follow any of a virtually unlimited number of pathways. The effect is to give humans a seemingly infinite variety of responses to sensory input, which may depend upon experience, mood, or any of numerous other factors. During both sleep and consciousness, the ceaseless electrochemical activity in the brain generates brain waves that can be electronically detected and recorded (see electroencephalography electroencephalography (əlĕk'trōĕnsĕf'əlŏg`rafē)
..... Click the link for more information. ).
Research
Brain research, now often referred to as a part of neuropsychology, cognitive science, psychobiology, or other similar fields, has become much more active in recent years. Aided largely by advanced new imaging techniques such as MRI (magnetic resonance magnetic resonance, in physics and chemistry, phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the
..... Click the link for more information. imaging) and the PET (positron emission tomography) scan, neuroscientists have been better able to localize specific functions involving thought, language, perceiving, mental imaging, memory, and other abilities. Much more has been learned about the roles of neurotransmitters neurotransmitter, chemical that transmits information across the junction ( synapse ) that separates one nerve cell (neuron) from another nerve cell or a muscle. Neurotransmitters are stored in the nerve cell's bulbous end (axon).
..... Click the link for more information. as well. New life has been given to the traditional philosophical debate on how to reconcile the seeming contradiction between the richness of subjective experience, including self-awareness, with purely scientific explanations of brain function.
Bibliography
See D. Dennett, Consciousness Explained (1991); J. A. Hobson, The Chemistry of Conscious States (1994); S. A. Greenfield, The Human Brain (1997); M. R. W. Dawson, Understanding Cognitive Science (1998); J. M. Allman, Evolving Brains (1999).
brain
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brain
Brain
A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. The three main parts of the brain in vertebrates are the cerebrum, the cerebellum, and the brainstem that connects them with each other and with the spinal cord (see illustration). The two cerebral hemispheres are separated by a midline fissure that is bridged by a massive bundle of axons running in both directions, the corpus callosum. Each hemisphere has a core of groups of neurons (the basal ganglia); an outer shell of neurons in layers (the cerebral cortex); and massive bundles of axons for communication within the cerebrum and with the rest of the brain. These bundles are called white matter because of the waxy myelin sheaths surrounding the axons.
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The basal ganglia comprises three main groups. (1) The thalamus receives axons from all sensory systems and transmits information to the cortex. It also receives feedback from cortical neurons during sensory processing. (2) The striatum, comprising bundles of axons cutting through the groups of neurons, also has two-way communication with the cortex and assists in the organization of body movement. (3) The hypothalamus receives orders from the cortex and organizes the chemical systems that support body movement. One output channel is hormonal, and controls the pituitary gland (hypophysis) which in turn controls the endocrine system. The other channel is neural, comprising axons coursing through the brainstem and spinal cord to the motor neurons of the autonomic nervous system, which regulates the heart, blood vessels, lungs, gastrointestinal tract, sex organs, and skin. The autonomic and endocrine systems are largely self regulating, but they are subject to control by the cortex through the hypothalamus. See Autonomic nervous system, Endocrine system (vertebrate), Neurobiology
The cortex is also called gray matter because it contains the axons, cell bodies, and dendrites of neurons but there is very little myelin. An index of the capacity of a brain is cortical surface area. In higher mammals, the cortical surface increases more rapidly than the volume during fetal development; as a result the surface folds, taking the form of convexities (gyri) and fissures (sulci) that vary in their details from one brain to another. However, they are sufficiently reliable to serve as landmarks on the cerebral hemisphere that it can be subdivided into lobes. Four lobes make up the shell of each hemisphere, namely the frontal, parietal, temporal, and occipital lobes. Each lobe contains a motor or sensory map (an orderly arrangement of cortical neurons associated with muscles and sensory receptors on the body surface). The central sulcus delimits the frontal and parietal lobes. The precentral gyrus contains the motor cortex whose neurons transmit signals to motor neurons in the brainstem and spinal cord which control the muscles in the feet, legs, trunk, arms, face, and tongue of the opposite side of the body. The number of neurons for each section is determined by the fineness of control, not the size of the muscle; for example, the lips and tongue have larger areas than the trunk. Within the postcentral gyrus is the primary somatosensory cortex. Sensory receptors in the skin, muscles, and joints send messages to the somatosensory cortical cells through relays in the spinal cord and the thalamus to a map of the opposite side of the body in parallel to the map in the motor cortex. The lateral fissure separates the temporal lobe from the parietal and frontal lobes. The cortex on the inferior border of the fissure receives input relayed through the thalamus from the ears to the primary auditory cortex. The occipital lobe receives thalamic input from the eyes and functions as the primary visual cortex.
In humans, the association cortex surrounds the primary sensory and motor areas that make up a small fraction of each lobe. The occipital lobe has many specialized areas for recognizing visual patterns of color, motion, and texture. The parietal cortex has areas that support perception of the body and its surrounding personal space. Its operation is manifested by the phenomenon of phantom limb, in which the perception of a missing limb persists for an amputee. Conversely, individuals with damage to these areas suffer from sensory neglect. The temporal cortex contains areas that provide recognition of faces and of rhythmic patterns, including those of speech, dance, and music. The frontal cortex provides the neural capabilities for constructing patterns of motor behavior and social behavior. It was the rapid enlargement of the frontal and temporal lobes in human evolution over the past half million years that supported the transcendence of humans over other species. This is where the capacity to create works of art, and also to anticipate pain and death, is located. Insight and foresight are both lost with bilateral frontal lobe damage, leading to reduced experience of anxiety, asocial behavior, and a disregard of consequences of actions.
A small part of frontal lobe output goes directly to motor neurons in the brainstem and spinal cord for fine control of motor activities, such as search movements by the eyes, head, and fingers, but most goes either to the striatum from which it is relayed to the thalamus and then back to the cortex, or to the brainstem from which it is sent to the cerebellum and then through the thalamus back to the cortex. In the cerebellum, the cortical messages are integrated with sensory input predominantly from the muscles, tendons, and joints, but also from the eyes and inner ears (for balance) to provide split-second timing for rapid and complex movements. The cerebellum also has a cortex and a core of nuclei to relay input and output. Their connections, along with those in the cerebral cortex, are subject to modification with learning in the formation of a working memory (the basis for learned skills). See Memory, Motor systems
The cerebellum and striatum do not set goals, initiate movements, store temporal sequences of sensory input, or provide orientation to the spatial environment. These functions are performed by parts of the cortex and striatum deep in the brain that constitute another loop, the limbic system. Its main site of entry is the entorhinal cortex, which receives input from all of the sensory cortices, including the olfactory system. The input from all the sensory cortices is combined and sent to the hippocampus, where it is integrated over time. Hippocampal output returns to the entorhinal cortex, which distributes the integrated sensory information to all of the sensory cortices, updates them, and prepares them to receive new sensory input. This new information also reaches the hypothalamus and part of the striatum (the amygdaloid nucleus) for regulating emotional behavior. Bilateral damage to the temporal lobe including the hippocampus results in loss of short-term memory. Damage to the amygdaloid nucleus can cause serious emotional impairment. The Papez circuit is formed by transmission from the hippocampus to the hypothalamus by the fornix, then to the thalamus, parietal lobe, and entorhinal cortex. The limbic system generates and issues goal-directed motor commands, with corollary discharge to the sensory systems that prepares them for the changes in sensory input caused by motor activity (for example, when one speaks and hears oneself, as distinct from another).
Each hemisphere has its own limbic, Papez, cortico-thalamic, cortico-striatal, and cortico-cerebellar loops, together with sensory and motor connections. When isolated by surgically severing the callosum, each hemisphere functions independently, as though two conscious persons occupied the same skull, but with differing levels of skills in abstract reasoning and language. The right brain (spatial)-left brain (linguistic) cognitive differences are largely due to preeminent development of the speech areas in the left hemisphere in most right- and left-handed persons. Injury to Broca's area (located in the frontal lobe) and Wernicke's area (located in the temporal lobe) leads to loss of the ability, respectively, to speak (motor aphasia) or to understand speech (sensory aphasia). Studies of blood flow show that brain activity during intellectual pursuits is scattered broadly over the four lobes in both hemispheres. See Aphasia, Central nervous system, Hemispheric laterality
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